As described in the article, the Walled Lake Police well and truly screwed the pooch on this one:

The day before Charles Simkins shot and killed his neighbor over a dog, police went to his Walled Lake home on a complaint that he was threatening an animal control officer.

On that December afternoon, they found the Iraq war veteran in a rage. He had an illegal Glock handgun in a holster at his hip, screamed and cursed at police, and at one point, stripped to the waist — despite subfreezing temperatures. Simkins was furious they were questioning him about his dogs and the gun, which he insisted he had a right to carry.

As they tried to reason with him, he knelt bare-chested in the snow, threatening to expose his genitals and begging to be arrested. Worried about their safety, officers snatched the handgun out of his holster and told him they were confiscating it, further enraging him.

But they did not arrest him that day on the gun charge and instead left the scene. Nor did they run a background check that would have showed he had an additional 22 registered handguns stored in his trash-filled home on Sigma Street. He used one of those guns 33 hours later to kill neighbor Edwin Criswell.

Really, you don't take someone in after they threaten an animal control officer while armed, kneel in the snow and beg to be arrested? Especially when you have an easy arrest for the threats to the animal control officer and carrying concealed in a vehicle when his CPL was expired?

Did no one at the scene think this is abnormal behavior, even for and most especially in Walled Lake where people are pretty boringly normal for the most part?

No one checked to see if he had other firearms, and he's not arrested after the police were worried enough that he was threatening them to take his firearm from him?

Even better they didn't consider an armed threat and his behavior to be a danger to himself or others?